Pulley/Transcript
Transcript Text reads: The Mysteries of Life with Tim and Moby. A robot, Moby, pulls open window blinds to reveal a bright blue sky outside the window. TIM: Mmmmmm… Good morning. A man, Tim, is lying in bed. Tim opens his eyes and then reads from a piece of paper. TIM: Dear Tim and Moby, I need an example of a pulley. From, Jimmy The letter is signed “Jimmy, via the Internet.” TIM: Well, the pulley is what’s called a simple machine, and simple machines like the lever, wheel and axle, inclined plane, and pulley are the most basic of tools. Tim is facing the screen. As he speaks, illustrations of the simple machines he names appear in insets. The lever is a bar leaning on a round object so that one will go up as the other end goes down. The wheel and axle are shown in two insets: a side view that shows the wheel is shaped like a circle, and a front view showing two wheels connected by a bar, or the axle. The inclined plane is drawn as a right triangle. The pulley is a round object hanging from a support, with two lines coming down and out that represent cable or rope. TIM: They each have one basic function. They’re simple. When you want to open a window blind, you pull down on a cord. Your pull is called the effort force. The window blinds are closed again. While Tim is speaking, Moby’s hand reaches out and pulls a cord down, raising the blinds to show a cloud in the sky. Moby let’s go, and his hand is replaced by a large downward-pointing arrow. TIM: The weight of the load exerts the resistance force. Another downward-pointing arrow is drawn inside the window. TIM: The blind has a wheel fixed to a support. This is our pulley. A close-up of the pulley is shown. TIM: A rope runs over the wheel and attaches to the blind. The same pulley is shown from farther away, initially without the rope. It is mounted in a window with blinds. The vertical bar supporting the pulley is attached to the top of the window frame. The rope is drawn in along its path. TIM: The blind is the “load” that you want to move. Pulling down on the rope can lift the load as high as its support. The scene zooms out further so the entire width of the window is visible. An arrow points down and to the right, parallel to the diagonal end of the rope. During the narration, the arrow moves along the direction it is pointing, and the blinds begin to rise. The rope is attached to the bottom of the blinds. Another arrow, pointing up, is drawn near the point where the rope attaches to the blinds. It moves up with the blinds. MOBY: Beep. Tim and Moby are standing next to each other. Tim is wearing a tee-shirt with a drawing of three pulleys sharing a single rope. TIM: Well, it wouldn’t be as easy if you had to stand on the windowsill and lift that blind by yourself. MOBY: Beep. TIM: You don’t think so, huh? Um… Try lifting that boulder over there. Moby crouches down and tries to lift a boulder about the same size as himself. He does not move the boulder. TIM: Pulleys work by changing the direction of the effort force and letting you use your body’s weight to support that effort force. One end of a rope is tossed over a tree branch. The loose end falls slack initially, and then straightens outward as if under tension. Then the rope is pulled along its length so that one end is going up and the other end is going down. TIM: Single pulleys, including our makeshift pulley here, have a mechanical advantage of one. Tim is talking in the foreground while Moby is in the background using the rope and tree branch to lift the boulder he tried to lift before. One end of the rope is tied around the boulder, and Moby is pulling the other end. Moby is raising the boulder slowly. TIM: For every meter Moby pulls the rope, the rock raises one meter. Two arrows of the same length are drawn onto the scene. One arrow, near and parallel to Moby’s end of the rope, is pointing diagonally down. The other arrow, near and parallel to the boulder’s end of the rope, is pointing up. TIM: That means that the effort force and the resistance force are the same. MOBY: Beep. TIM: Well, that’s true…some of the effort force is used to overcome friction. A close up view of the rope and tree branch shows some frayed threads at the point where the rope is rubbing along the tree branch as it moves. TIM: A single pulley doesn’t increase your effort force, so if an object is heavier than you are, you won’t be able to lift it. Moby is now using a pulley to lift a block that is much larger than him. Moby is pulling back, but the large block is not moving. TIM: But if you have a double-pulley system where one pulley is attached to a fixed support, and the other is attached to the load, a person (or a robot) can lift a load several times his or her weight! The top of the large block is shown along with two pulleys. The first pulley is at the top of the screen and attached to something that is above the view. A rope passes through the first pulley, moving down to a second pulley. The second pulley is mounted on the large block. Initially, the rope appears to be using the second pulley as a point of attachment, because it ends at the second pulley. During the narration, the top pulley shifts over to the right, and a third pulley appears at the top and to the left. A third segment of the rope from the second pulley to the third pulley becomes visible. Now the rope appears to be using the second pulley as a pulley, and the third pulley as an attachment. The block begins to rise. TIM: With one fixed and one movable wheel at work, the force raising the load doubles the effort force. The scene zooms enough to see that Moby is pulling down on the rope to lift the large block. TIM: The effort force needed is now one-half the resistance force. Equation reads: effort force equals StartFraction one-half EndFraction resistance force. TIM: Moby now has to pull the rope two meters to get the weight to move one meter, but it’s much easier to do because he spreads his effort out over those two meters of rope. The view changes to show Moby raising the block from a different angle. Moby is pulling the rope down a longer distance than the block is going up. TIM: The amount of work always remains the same. When you decrease your effort force, you have to exert that force over twice as much rope. With work, you always get what you pay for. Moby is trying to raise a car with a pulley system like the one he used on the block. So far, the car has tilted up so that it is partially in the air, but one corner is still on the ground. Moby grins. TIM: Oh, that’s not going to be good. Category:BrainPOP Transcripts Category:BrainPOP Engineering & Technology Transcripts